The long-term goal of this research is to gain knowledge about the mechanisms which determine the radiosensitivity of mammalian cells, particularly in reference to those involved in the response of stationary or very slowly proliferating cells to x-irradiation. We will utilize density inhibited plateau phase cultures of an established line of human cells. Cells in these cultures will repair potentially lethal radiation damage if allowed to remain in a resting phase of growth for several hours after irradiation. The cell age dependence of the repair processes will be determined, as well as the relation of repair to progression through the cell cycle in synchronized cultures. This cellular repair phenomenon will be correlated with evidence for molecular DNA repair processes such as unscheduled DNA synthesis, and the rejoining of single strand breaks as determined by alkaline sucrose gradient techniques. The nutritional factors including oxygenation which govern the growth rate and density inhibition of cells will also be studied and correlated with radiosensitivity and repair capability. Somatic cell hybridization techniques will be employed to obtain mutant mammalian cells with differing radiosensitivities and repair capacities. In all of these studies, an overall objective is to understand and correlate events such as repair at the molecular level with actual changes in cell survival.